JP2001-280485 discloses a control that causes a friction element to slip to bring the friction element into close to a neutral state when a vehicle stops while a shift lever is in a traveling range. When executing such a control, JP2001-280485 learns and corrects an engaging force command value of the friction element.
During a vehicle stop, in a variator, a speed ratio of the variator is maintained at a speed ratio on a Lowest side (hereinafter, referred to as a “Lowest speed ratio”) for a next vehicle start. Then, when the speed ratio is not the Lowest speed ratio during the vehicle stop, the speed ratio is shifted to the Lowest speed ratio from the speed ratio during the vehicle stop. In this case, a target speed ratio is set to a largest value (hereinafter, referred to as a “mechanical largest speed ratio”) of the speed ratio, which is on a further Low side with respect to the Lowest speed ratio and can be mechanically taken by the variator, such that an actual speed ratio becomes the Lowest speed ratio even when a variation and the like are present. Setting the target speed ratio to this mechanical largest speed ratio enables to cause the actual speed ratio to be close to the Lowest speed ratio regardless of the variation to ensure reducing a driving force shortage at the time of the vehicle start.
However, there is a case where a hydraulic pressure supplied to the variator becomes low during the vehicle stop and the actual speed ratio of the variator cannot be brought into the Lowest speed ratio. This, for example, can happen when an engine rotational speed is lowered with the vehicle stop to cause a decreased amount of oil discharged from an oil pump driven by being transmitted the rotation of the engine. In such a case, in order to cause the actual speed ratio to be close to the target speed ratio (the mechanical largest speed ratio when the target speed ratio is the mechanical largest speed ratio), a lowering command of a primary pulley pressure or an increasing command of a secondary pulley pressure are continued to be output by a feedback control. When such commands are output, a friction in the variator varies.
Thus, when the friction of the variator varies, an input rotational speed of the variator, in other words, a turbine rotational speed of a torque converter varies. Then, when the friction in the variator increases in a state where the friction engaging element is disengaged in a vehicle where the friction engaging element, such as a clutch, is disposed between the variator and a driving wheel, a load in the variator increases; therefore, the turbine rotational speed is lowered. On the other hand, when the friction in the variator decreases in a similar state, the load in the variator lowers; therefore, the turbine rotational speed is increased.
Learning and correcting the above-described engaging force command value is typically performed on the basis of the turbine rotational speed; therefore, when the friction of the variator varies and the turbine rotational speed varies during the learning, the learning and correcting cannot be stably executed, thus deteriorating a learning accuracy.
Then, in one aspect of the present invention, a control device that controls an automatic transmission is provided, in which device the automatic transmission includes a variator disposed in a power transmission path between a driving source and a driving wheel of a vehicle, and a friction engaging element disposed between the variator and the driving wheel, in a manner capable of transmitting a power disconnectably via the power transmission path. In this aspect, a speed ratio of the variator is increased toward a predetermined target speed ratio with disengaging the friction engaging element during a vehicle stop of the vehicle, and a learning regarding a hydraulic control of the friction engaging element is executed when the friction engaging element is disengaged during the vehicle stop. Furthermore, the target speed ratio is decreased at a time of learning when the learning is executed during the vehicle stop, compared to a time of vehicle stop other than the time of learning.
Furthermore, in another aspect of the present invention, a control method of controlling an automatic transmission is provided, in which method a variator is disposed in a power transmission path between a driving source and a driving wheel of a vehicle, and a friction engaging element is disposed between the variator and the driving wheel, in a manner capable of transmitting a power disconnectably via the power transmission path. In this aspect, a speed ratio of the variator is increased toward a predetermined target speed ratio with disengaging the friction engaging element during a vehicle stop of the vehicle, and a learning regarding a hydraulic control of the friction engaging element is executed when the friction engaging element is disengaged during the vehicle stop. Furthermore, the target speed ratio is decreased at a time of learning when the learning is executed during the vehicle stop, compared to a time of vehicle stop other than the time of learning.
With the above-described aspect, the learning regarding the hydraulic control of the friction engaging element can be accurately executed during the vehicle stop.